The present invention relates to devices and methods for shared-medium data-transmission networks, and particularly, using forward error correction (FEC) coding in Ethernet networks, as specified in the IEEE 802.3, clause 65.
Ethernet technology is one of the most common digital-networking technologies. Specified in the Institute of Electrical and Electronics Engineers (IEEE) standard 802.3, the technology has a large installed base of compatible network devices. (The 2005 edition of the IEEE 802.3 standard is filed together with this document and is hereby incorporated by reference as if fully set forth herein.) Ethernet technology continues to evolve with newer and faster variants, such as the Gigabit Ethernet, which provides network speeds of one gigabit per second.
For several decades, Ethernet technology has been widely used in local-area networks (LANs). More recently, Ethernet technology has been used with increasing frequency in metro-area network (MAN) and other wide-area network (WAN) applications. Such networks include optical networks, such as passive optical networks (PONs). In WAN applications, signal attenuation and budget constraints on the network link have greater importance because of the distances involved for the WAN connectivity, and because of the optical power splitters that may be used along a network link between a transmitter and a receiver.
At some point along the network link, a signal may be attenuated and distorted to such a degree that the information the signal carries cannot be extracted because of: the limited sensitivity of the receiver, noise in the propagation medium, signal source-related noise (such as inter-symbol interference and mode-partition noise), and other sources of noise, attenuation, and distortion. But long before this point is reached, the signal-to-noise ratio (SNR) of the signal deteriorates, and the bit error rate (BER) of the signal increases beyond the tolerable level for a typical application.
Forward Error Correction (FEC) is one method for improving the BER of a received signal with low SNR. The operation of FEC is described in Khermosh, US Patent Publication No. 20050005189 (assigned to the assignee of the present invention and henceforth referred to as Khermosh '189), which patent application is incorporated by reference for all purposes as if fully set forth herein. FEC is a coding technique that uses additional (i.e. redundant, or parity-check) symbols as part of a transmission of a digital signal sequence through a physical channel. The symbols are a type of error control codes. But, because of the presence of sufficient redundancy, when errors corrupt the received signal, the receiver not only recognizes the errors, but also corrects the errors without requesting retransmission. In practice, the improvement in the BER achieved through the use of FEC is known as “coding gain.”
Adding FEC capability to a legacy Ethernet network may cause errors in the media access control (MAC) layer of the non-FEC-capable (i.e. legacy) network elements. Moreover, applying FEC only to the payloads of data packets would not affect network-link budget constraints because packet headers, which carry destination information and frame boundary fields of the packets, would not benefit from the improved BER of the payloads. On the other hand, applying FEC separately to the headers and payloads can make the headers unrecognizable to the “non-FEC” network elements. These headers should be decoded in a reliable manner to provide line immunity. Furthermore, the headers should be decoded more reliably than the FEC protection so as not to limit network performance.
The method of packet-based FEC is specified in the IEEE 802.3, clause 65. Specific FEC code of Reed Solomon (RS) (255,239,8) code was selected therein, and specific S_FEC and T_FEC delimiters were selected for the line. The delimiters are a sequence of 8-bit/10-bit (8B/10B in 1000BaseX) code symbols. The 8B/1B code is the physical coding sub-layer (PCS) line code of the 1G Ethernet as specified in the IEEE 802.3, clause 36.
It would be desirable to have devices and methods for improving BER for a given network link budget, and conversely, for increasing the network link budget for a given BER, on Ethernet-compatible networks having non-FEC-capable legacy network elements.